Alterman launched his own journalism career in 1983 as a freelancer for The Nation, The Washington Monthly, The New Republic, Harper's, and Le Monde Diplomatique. He later wrote for such publications as Vanity Fair, The New York Times Magazine, The New Yorker, and The Atlantic Monthly, while working as a Senior Fellow for the World Policy Institute. After that, Alterman was a Washington Correspondent for Mother Jones and Rolling Stone, before returning to The Nation as a columnist in 1995.

During the mid- to late 1980s, Alterman was a staffer with the Guardian, a now-defunct Maoist journal affiliated with the Institute for Policy Studies (IPS). On a brochure for IPS's 30th Anniversary celebration in 1993, Alterman was listed as one of that organization's “former Visiting Fellows and Visiting Scholars,” and also as a “current … Fellow” of the TransNational Institute, which was a major source of anti-American, anti-capitalist propaganda.

In 1996, MSNBC hired Alterman as a television commentator and Internet columnist. In 2002 Alterman created Altercation—a daily blog about media, politics, and culture—for MSNBC's website. In September 2006 he ended his association with MSNBC and was hired as a Senior Fellow by Media Matters for America, which also took over the sponsorship of Altercation. On December 22, 2008, Alterman announced that in 2009 Altercation would be moving to The Nation's website, where it would appear “more sporadically” than it had in the past. In addition to Altercation, Alterman also writes a regular “Think Again” column for the Center for American Progress, where he is currently a Senior Fellow.

In April 2003, Alterman was a signatory to a Statement on Cuba, a petition that was initiated and circulated by Leo Casey, a Marxist member of the Democratic Socialists of America. The Statement called for the lifting of United States trade sanctions against Cuba, and it blamed Cuba's economic and political problems in part on “reactionary elements of the U.S. administration.”

Strongly opposed to the 2003 U.S. invasion of Iraq, in May of that year Alterman claimed that the American government’s alleged concern about Saddam Hussein’s possession of weapons of mass destruction “was all a big joke; something for suckers to get worked up about while [President] Bush and company went to war for their own reasons.” Alterman pronounced, further, that“the Likud faction of the Republican Party would like a war without any bothersome discussion beforehand.” He elaborated on this theme in September 2004, with the added suggestion that the Iraq War may have been motivated in large measure by pressure from the Israel lobby. Among Alterman's assertions were that:

“The American people were purposely misled and are paying for it dearly, in both blood and treasure.”

“The war was planned by neoconservatives, many of whom worked directly with their counterparts in the Israeli government, who helped perpetuate the deception.”

“The war did improve the security of Israel, but not that of the United States. No other country's population thought it was a good idea, including Britain, save that of Israel.”

“Pentagon neocons were spying for Israel and using the Israel lobby as a conduit.”

During the run-up to the 2004 presidential election, Alterman publicly lauded the billionaire philanthropist George Soros for having recently announced his intention to spend $15.5 million to help the Democratsdefeat President George W. Bush at the polls.

When the British Muslim Council (BMC) decided in 2005 to boycott a ceremony commemorating the 60th anniversary of the liberation of Auschwitz because the event did not honor the Palestinian victims of Israeli “genocide,” Alterman supported the BMC's decision. “I'm a Jew,” he wrote, “but I don't expect Arabs to pay tribute to my people's suffering while Jews, in the form of Israel and its supporters … are causing much of theirs.” Further, Alterman noted that: “The Palestinians have also suffered because of the Holocaust. They lost their homeland as the world—in the form of the United Nations—reacted to European crimes by awarding half of Palestine to the Zionists.... To ask Arabs to participate in a ceremony that does not recognize their own suffering but implicitly endorses the view that caused their catastrophe is morally idiotic."

In October 2006, Alterman was a signatory to a document titled “We Answer to the Name of Liberals,” which blamed the Bush administration for: launching the “illegal, unwise” Iraq War; having “discouraged the prospects for an honorable Israeli-Palestinian settlement”; its excessive “reliance on military intervention”; its “contempt” for “the rule of law”; its “vast tax cuts to the rich at the expense of policies that strengthen the common ties that bind us together as a community”; its “suppression of votes” at the polls; its “shameful” environmental record; its numerous “human-rights violations”; its “rejection of basic guarantees of due process”; and its failure to protect Americans “from foreign enemies on [9/11], and from the hurricane [Katrina] and flood that afflicted the Gulf Coast in 2005.” This document was co-written by Bruce Ackerman and Todd Gitlin. Additional signers included such notables as Robert Reich, Ben Waxman, Daniel Okrent, Michael Berube, and Michael Tomasky.

Alterman was a protege and defender of the late author and journalist I.F. Stone. When historians John Earl Haynes, Harvey Klehr, and Alexander Vassiliev demonstrated in 2009 that Stone had been a Soviet agent who assisted the KGB during the mid to late 1930s, Alterman swiftly published a response article titled “I.F.Stone Was No Spy.” Altermancharacterized the Stone-KGB allegations as “phony,” “pathetic,” unfounded “smears” that were part of “an almost entirely bogus controversy over whether Stone ever willingly spied for the Russians or cooperated with the KGB in any way. He did not.” Alterman also claimed that “the campaign to smear Stone bears the hallmarks of a foundation-funded campaign of right-wing media manipulation.”

Alterman was an identified member of JournoList—a secret alliance of some 400 liberal and left-wing journalists, academics, and political activists who from 2007-10 colluded to discredit and bury stories that had the potential to harm Barack Obama's presidential campaign and political agendas.

In a 2012 interview with Bill Moyers, Alterman said that “as a society we've moved incredibly further to the right since [Franklin] Roosevelt's time”; that former President Richard Nixon was “more liberal than Barack Obama”; and that “Obama is really quite similar to Dwight Eisenhower, both in terms of the role of government and what it can accomplish.”

Impugning “members of the mainstream media” for giving “too much credence to empty claims of 'voter fraud',” Alterman has criticized “restrictive voter-ID laws that are transparently designed to reduce minority participation” in political elections.

Alterman was a signatory to September 2015 letter that a pair of coalitions called “Partners for Progressive Israel” and “Scholars for Israel and Palestine” sent to every member of Congress, urging the lawmakers to approve the nuclear agreement that the Obama administration and the governments of five other nations were negotiating with Iran. That accord, said Alterman and his fellow signers, would force Iran to: “forswea[r] further development of nuclear weapons, commi[t] to significant reductions in its current nuclear capabilities, and agre[e] to intrusive inspections” that “will greatly reduce the chances of war with Iran and will enhance the security of Israel.” (For details about what the Iran nuclear deal actually stated, click here.) Another noteworthy signatory of the letter to Congress was Todd Gitlin.